For many years there has been used in the coal mining industry continuous mining machines in which a plurality of generally conical bits known as a plumb bob bits are used. The body or base of each bit is an iron or steel casting or forging having a frustoconical front end nose portion provided with an axial hole in which a tungsten carbide insert is mounted. The tungsten carbide insert extends from the front end of the nose portion of the metal body and provides the cutting surface of the bit. The tungsten carbide inserts of the prior art had a diameter which was about one-third or less the minimum diameter of the frustoconical nose portion of the base.
In use a large number of such bits are mounted for free axial rotation in the outer surface of a rotary drum, or in the outer surface of a continuous chain or the like, and the bits are moved through an orbit which is intercepted by the face of the material being mined. The bits contact the material being mined at a small angle relative to their longitudinal axes, and the bits are thus rotated about their respective longitudinal axes as they travel across and through the material being mined. Such bits have had to be frequently removed and discarded or sharpened, and they have had a relatively short life primarily because the portions of the base material adjacent the tungsten carbide was worn away as the bits moved through the material being mined.
Another problem with the prior art bits has been the breakage of the sharp tips when the bits are initially put into use.
A serious problem in the coal mining industry has been the ignition of methane gas released from pockets where the gas has been trapped in the material being mined. It is believed that a primary cause of such ignition is the heat which is frictionally developed as the bits move through the coal and rock during the mining operation. It is also possible that the sparking which occurs when the metal base portions of the bits strike rock causes ignition of the methane gas.